Semiconductor memory device, information processing system including the same, and controller

ABSTRACT

To include first and second data input/output terminals allocated to first and second memory circuit units, respectively, and an address terminal allocated in common to these memory circuit units. When a first chip selection signal is activated, the first memory circuit unit performs a read operation or a write operation via the first data input/output terminal based on an address signal regardless of an operation of the second memory circuit unit. When a second chip selection signal is activated, the second memory circuit unit performs a read operation or a write operation via the second data input/output terminal based on the address signal regardless of an operation of the first memory circuit unit. With this configuration, a wasteful data transfer can be prevented, and the effective data transfer rate can be increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device and aninformation processing system including the same, and more particularlyrelates to a semiconductor memory device having a plurality of memorycircuit units operable independently of each other and an informationprocessing system including the semiconductor memory device. The presentinvention also relates to a controller that controls the semiconductormemory device.

2. Description of Related Art

Many DRAMs (Dynamic Random Access Memories) as representativesemiconductor memory devices have their internal portions divided intoplural banks in order to enable parallel operations (see Japanese PatentApplication Laid-open No. H11-66841). A controller connected to theDRAMs can individually issue a command to each bank, and when a certainbank is performing a read operation or a write operation, the controllercan issue commands to another bank. As a result, banks can performparallel operations, thereby increasing utilization efficiency of a databus connected between the DRAMs and the controller.

However, because these bank share a data input/output terminal, readdata cannot be output from another bank during a period when read datais being output from a certain bank. Therefore, even when a part of bitsof read data output from a certain bank is unnecessary for thecontroller, read data cannot be output from another bank until when aseries of burst output are finished.

In a so-called multibit product, a part of bits of read data is notnecessary in many cases. For example, when a DRAM has 32 bits for I/O(input and output) data, a controller requires only 16-bit data in manycases. In this case, the rest of 16 bits are invalidated by thecontroller. Frequent occurrence of such situations lowers utilizationefficiency of a data bus, resulting in a problem that an effective datatransfer rate is decreased.

FIG. 9 is a timing chart for explaining this problem.

FIG. 9 is an example of an operation of a DDR synchronous DRAM in whichI/O data has 32 bits (DQ0 to DQ31), a burst length is 4 (BL=4), and aCAS latency is 5 (CL=5). Meshed data is necessary data, and unmesheddata is unnecessary data. In this example, because the BL is 4, readcommands (A, B, C, and D) can be input at every two clock cycles.

At one-time access, 128-bit (=32×4) data is output from such a DRAM. Inthe example shown in FIG. 9, either 64-bit data output from DQ0 to DQ15or 64-bit data output from DQ16 to DQ31 is necessary data, and rest ofthe data is not necessary. In this case, because only a half of theoutput data is necessary, the effective data transfer rate decreases toa half.

While a problem in the read operation has been explained with referenceto FIG. 9, this problem also occurs in a write operation.

As described above, according to a conventional semiconductor memorydevice, when a part of read data or write data is unnecessary data, itseffective data transfer rate decreases. This kind of problem occursnoticeably in multibit products having a large number of I/O bits.

SUMMARY

In one embodiment, there is provided a semiconductor memory device thatincludes: first and second memory circuit units activated in response tofirst and second selection signals, respectively; first and second datainput/output terminals allocated to the first and second memory circuitunits, respectively; and an address terminal allocated in common to thefirst and second memory circuit units, wherein when the first selectionsignal is activated, the first memory circuit unit performs a readoperation or a write operation via the first data input/output terminalbased on an address signal input via the address terminal, regardless ofan operation of the second memory circuit unit, and when the secondselection signal is activated, the second memory circuit unit performs aread operation or a write operation via the second data input/outputterminal based on the address signal, regardless of an operation of thefirst memory circuit unit.

In another embodiment, there is provided an information processingsystem that includes: the above semiconductor memory device and acontroller that supplies at least the address signal and the first andsecond selection signals to the semiconductor memory device.

In still another embodiment, there is provided a controller that obtainsread data from a semiconductor memory device or writes write data intothe semiconductor memory device, by supplying an address signal to thesemiconductor memory device, wherein the controller individuallyaccesses a plurality of memory circuit units included in onesemiconductor memory device by supplying a plurality of chip selectionsignals to the semiconductor memory device.

As explained above, in the semiconductor memory device according to thepresent invention, a plurality of memory circuit units are provided anda data input/output terminal is allocated to each memory circuit unit.Therefore, read data can be output from a certain memory circuit unitwhile read data is being output from another memory circuit unit. Thisis also the same in a write operation. With this configuration, becauseunnecessary read data or unnecessary write data does not have to betransferred, utilization efficiency of a data bus can be increased.Furthermore, because an address terminal is common to a plurality ofmemory circuit units, increase of the number of terminals can beminimized.

Further, because the controller can handle one semiconductor memorydevice as plural chips, a data bus can be efficiently used withoutmaking an information processing system complex.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features and advantages of the present invention will be moreapparent from the following description of certain preferred embodimentstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic block diagram showing a configuration of asemiconductor memory device 100 according to a first embodiment of thepresent invention;

FIG. 2 is a block diagram showing a detailed configuration of thesemiconductor memory device 100 according to the first embodiment;

FIG. 3A is a block diagram of an information processing system 200 usingthe semiconductor memory device 100 according to the first embodiment;

FIG. 3B is a block diagram of a controller 210;

FIG. 4 is a timing diagram for explaining a read operation of thesemiconductor memory device 100 according to the first embodiment, andshows a case of alternately accessing the memory circuit units 110A and110B;

FIG. 5 is a timing diagram for explaining a read operation of thesemiconductor memory device 100 according to the first embodiment, andshows a case of simultaneously accessing the memory circuit units 110Aand 110B;

FIG. 6 is a timing diagram for explaining a refresh operation of thesemiconductor memory device 100 according to the first embodiment;

FIG. 7 is a block diagram showing a configuration of a semiconductormemory device 300 according to the second embodiment;

FIG. 8 is a timing diagram for explaining a read operation of thesemiconductor memory device 300 according to the second embodiment, andshows a case of alternately accessing the memory circuit units 110A and110B;

FIG. 9 is a timing chart for explaining a problem of a conventionalsemiconductor memory device; and

FIG. 10 is a block diagram of a semiconductor memory device havingplural banks.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will be explained belowin detail with reference to the accompanying drawings.

FIG. 1 is a schematic block diagram showing a configuration of asemiconductor memory device 100 according to a first embodiment of thepresent invention. A DDR synchronous DRAM is assumed as thesemiconductor memory device 100 according to the first embodiment.

As shown in FIG. 1, the semiconductor memory device 100 according to thefirst embodiment includes two memory circuit units 110A and 110B, and acommon circuit 120 allocated in common to these memory circuit units110A and 110B. The memory circuit units 110A and 110B are circuit blockscapable of mutually independently performing a read operation and awrite operation. A data input/output terminal group LDQ is allocated tothe memory circuit unit 110A, and a data input/output terminal group UDQis allocated to the memory circuit unit 110B. The data input/outputterminal group LDQ includes 16 data input/output terminals DQ0 to DQ15.The data input/output terminal group UDQ includes 16 data input/outputterminals DQ16 to DQ31.

In this way, the semiconductor memory device 100 according to the firstembodiment is a single memory (a memory integrated on a singlesemiconductor substrate) having 32 bits (DQ0 to DQ31) for I/O. A half ofthe data input/output terminals (DQ0 to DQ15) are allocated to thememory circuit unit 110A, and the rest half of the data input/outputterminals (DQ16 to DQ31) are allocated to the memory circuit unit 110B.Therefore, from a controller, it looks as if two memory chips arepresent. In this respect, the semiconductor memory device 100 is clearlydistinguished from a semiconductor memory device which is simply dividedinto plural banks.

On the other hand, an address terminal group 131 and a command terminalgroup 132 are common to the memory circuit units 110A and 110B. Anaddress signal ADD and a command signal CMD supplied via these terminalsare input to the common circuit 120. Therefore, from the controller,although it looks as if two memory chips are present, completelyindependent two memories are not actually integrated in one chip.Consequently, the memory chip is not different from a conventionalmemory chip having 32 bits for I/O, except that the number of terminalsis different in that a chip-selection-signal input terminal describedlater is added. On the other hand, when completely independent twomemories are simply integrated into one chip, this substantiallyincreases the number of terminals to almost twice. Therefore, in thisrespect, the present invention is clearly distinguished from a simpleintegration of two completely independent memories into one chip.

As shown in FIG. 1, the common circuit 120 has an address input circuit121 to which the address signal ADD is input, and a command inputcircuit 122 to which the command signal CMD is input. The command signalCMD is expressed by a combination of a row-address strobe signal RASB, acolumn-address strobe signal CASB, and a write enable signal WEB and soon. The address signal ADD and the command signal CMD input to theseinput circuits 121 and 122 are supplied to either one or both of thememory circuit units 110A and 110B. A selecting circuit 123 contained inthe common circuit 120 selects a memory circuit unit.

Chip selection signals CS1B and CS2B are input to the selecting circuit123 via chip-selection-signal input terminals 141 and 142, respectively.The chip selection signal CS1B is a signal to select the memory circuitunit 110A. When the chip selection signal CS1B is activated at a lowlevel, the address signal ADD and the command signal CMD input to theinput circuits 121 and 122 are supplied to the memory circuit unit 110A.On the other hand, the chip selection signal CS2B is a signal to selectthe memory circuit unit 110B. When the chip selection signal CS2B isactivated at a low level, the address signal ADD and the command signalCMD input to the input circuits 121 and 122 are supplied to the memorycircuit unit 110B. Therefore, when both of the chip selection signalsCS1B and CS2B are active, the address signal ADD and the command signalCMD are supplied to both of the memory circuit units 110A and 110B.

The memory circuit unit 110A has a memory cell array 111A includingplural word lines WL, plural bit lines BL, and plural memory cells MCarranged at intersections between these lines. A row decoder 112Aselects the word line WL included in the memory cell array 111A. Acolumn decoder 113A selects the bit line BL included in the memory cellarray 111A. The row decoder 112A selects the word line WL based on theaddress signal ADD supplied when the command signal CMD indicates anactive command. On the other hand, the column decoder 113A selects thebit line BL based on the address signal ADD supplied when the commandsignal CMD indicates a column command (a read command or a writecommand).

The memory cells MC selected by the row decoder 112A and the columndecoder 113A are connected to an input/output circuit 114A. With thisconnection, when the command signal CMD indicates a read operation, readdata read from the memory cell array 111A is output from the datainput/output terminal group LDQ (DQ0 to DQ15) via the input/outputcircuit 114A. When the command signal CMD indicates a write operation,write data input from the data input/output terminal group LDQ (DQ0 toDQ15) are written into the memory cell array 111A via the input/outputcircuit 114A.

A circuit configuration and an operation of the memory circuit unit 110Bare similar to the circuit configuration and the operation of the memorycircuit unit 110A, and thus redundant explanations thereof will beomitted.

When the chip selection signal CS1B is activated based on the aboveconfiguration, the memory circuit unit 110A performs a read operation ora write operation via the data input/output terminal group LDQ (DQ0 toDQ15) based on the address signal ADD input via the address terminal131, regardless of an operation of the memory circuit unit 110B.Similarly, when the chip selection signal CS2B is activated, the memorycircuit unit 110B performs a read operation or a write operation via thedata input/output terminal group UDQ (DQ16 to DQ31) based on the addresssignal ADD input via the address terminal 131, regardless of anoperation of the memory circuit unit 110A.

As explained above, while the conventional semiconductor memory devicehaving 32 bits for I/O needs to input and output data in 32 bits, thesemiconductor memory device 100 according to the first embodiment caninput and output data in 16 bits. Therefore, unnecessary read data orunnecessary write data is not required to be transferred, and thusutilization efficiency of a data bus can be increased.

FIG. 2 is a block diagram showing a detailed configuration of thesemiconductor memory device 100 according to the first embodiment.

As shown in FIG. 2, the common circuit 120 of the semiconductor memorydevice 100 according to the first embodiment further includes a commanddecoder 124, a mode register 125, a clock generating circuit 126, and aDLL (Delay Lock Loop) circuit 127. The selecting circuit 123 shown inFIG. 1 is divided into a clock control circuit 123A allocated to thememory circuit unit 110A, and a clock control circuit 123B allocated tothe memory circuit unit 110B.

The command decoder 124 generates an internal command ICMD by decodingthe command CMD input via the command input circuit 122. The generatedinternal command ICMD is supplied to the memory circuit units 110A and110B, as well as to the mode register 125.

The mode register 125 sets an operation mode of the semiconductor memorydevice 100. In the first embodiment, operation modes of the memorycircuit units 110A and 110B are set common by the mode register 125. ACAS latency (CL) and a burst length (BL) are mentioned as operationmodes set to the mode register 125. A set value of the mode register 125is updated based on the address signal ADD when the command signal CMDindicates “mode register set”.

The clock generating circuit 126 generates internal clocks ICLK and PCLKby receiving external clock signals CK and CKB supplied from outside.Among these clocks, the internal clock ICLK is supplied to the clockcontrol circuits 123A and 123B. The clock control circuit 123A generateslatch clocks CLKA, CLKAA, and CLKCA when the chip selection signal CS1Bis active. The latch clock CLKAA is an operation clock of an addresslatch circuit 112RAA included in the memory circuit unit 110A, and thelatch clock CLKCA is an operation clock of an address latch circuit112CAA and a command latch circuit 112CMA. With this arrangement, latchoperations of the address latch circuits 112RAA and 112CAA and thecommand latch circuit 112CMA included in the memory circuit unit 110Aare permitted only when the chip selection signal CS1B is active.

The address signal ADD latched by the address latch circuits 112RAA and112CAA can be a signal not decoded at all or can be a partly decodedpredecoded signal.

Similarly, the clock control circuit 123B generates latch clocks CLKB,CLKAB, and CLKCB when the chip selection signal CS2B is active. Withthis arrangement, latch operations of the address latch circuits 112RABand 112CAB and the command latch circuit 112CMB included in the memorycircuit unit 110B are permitted only when the chip selection signal CS2Bis active.

An OR circuit 128 logically adds the latch clocks CLKA and CLKBgenerated by the clock control circuits 123A and 123B, and supplies alatch clock CLK as a result of this OR operation, to the address inputcircuit 121 and the command input circuit 122. With this arrangement,latch operations by the address input circuit 121 and the command inputcircuit 122 are permitted when at least one of the chip selectionsignals CS1B and CS2B is active.

On the other hand, the internal clock PCLK is supplied to the DLLcircuit 127. The DLL circuit 127 generates an internal clock LCLK whichis phase-controlled to the external clocks CK and CKB. The generatedinternal clock LCLK is supplied in common to the input/output circuits114A and 114B included in the memory circuit units 110A and 110B. Theinternal clock LCLK is a signal to control an output timing of readdata. With this arrangement, the DLL circuit 127 controls an outputtiming of read data via the data input/output terminal groups LDQ andUDQ allocated to the memory circuit units 110A and 110B.

The memory circuit units 110A and 110B are explained next.

As described above, the memory circuit unit 110A includes the addresslatch circuits 112RAA and 112CAA, and the command latch circuit 112CMA.The address latch circuit 112RAA latches a row address RA out of theaddress signal ADD input via the address input circuit 121. The addresslatch circuit 112RAA performs a latch operation based on the latch clockCLKAA. The address latch circuit 112CAA latches a column address CA outof the address signal ADD input via the address input circuit 121. Theaddress latch circuit 112CAA performs a latch operation based on thelatch clock CLKCA. Further, the command latch circuit 112CMA latches theinternal command ICMD as output of the command decoder 124. The commandlatch circuit 112CMA performs a latch operation based on the latch clockCLKCA.

The row address RA latched by the address latch circuit 112RAA issupplied to the row decoder 112A via a row control buffer 115A, therebyselecting the word line WL. A column address CA latched by the addresslatch circuit 112CAA is supplied to the column decoder 113A via a columncontrol buffer 116A, thereby selecting a sense amplifier included in asense amplifier array 111 sA (that is, selecting the bit line BL).

Further, the internal command ICMD latched by the command latch circuit112CMA is supplied to a command control circuit 117A. The commandcontrol circuit 117A controls a data control circuit 118A and a datalatch circuit 119A, thereby controlling a transfer timing of read dataand write data.

The memory circuit unit 110B has the same circuit configuration as thatof the memory circuit unit 110A, except that the latch clocks CLKAB andCLKCB are used instead of the latch clocks CLKAA and CLKCA, and thusredundant explanations thereof will be omitted.

The circuit configuration of the semiconductor memory device 100according to the first embodiment is as described above. As explainedabove, the semiconductor memory device 100 according to the firstembodiment has a characteristic such that the device has twochip-selection-signal input terminals. Therefore, the controller thatcontrols the semiconductor memory device 100 can handle chips as twomemory chips that can be changed by the chip selection signals CS1B andCS2B.

FIG. 3A is a block diagram of an information processing system 200 usingthe semiconductor memory device 100 according to the first embodiment.

The information processing system 200 shown in FIG. 3A is configured bythe semiconductor memory device 100 according to the first embodimentand a controller 210 connected to the semiconductor memory device 100.The controller 210 and the semiconductor memory device 100 are connectedto each other by a command/address bus 220, data buses 230L and 230U,and selection buses 240L and 240U.

The command/address bus 220 is a wiring to supply the command signalCMD, the address signal ADD, and the external clocks CK and CKB from thecontroller 210 to the semiconductor memory device 100.

The data bus 230L is a wiring connected to the data input/outputterminal group LDQ (DQ0 to DQ15), and is used to transfer read data orwrite data of 16 bits between the controller 210 and the semiconductormemory device 100. The data bus 230U is a wiring connected to the datainput/output terminal group UDQ (DQ16 to DQ31), and is used to transferread data or write data of 16 bits between the controller 210 and thesemiconductor memory device 100.

The selection bus 240L is a wiring to supply the chip selection signalCS1B from the controller 210 to the semiconductor memory device 100. Theselection bus 240U is a wiring to supply the chip selection signal CS2Bfrom the controller 210 to the semiconductor memory device 100.

As explained above, two selection buses are used in the informationprocessing system 200.

FIG. 3B is a block diagram of the controller 210.

As shown in FIG. 3, the controller 210 includes command terminals 301,address terminals 302, chip select terminals 303-1 and 303-2, and datainput/output terminals 304UDQ and 304LDQ. The command terminals 301 aresupplied with a command signal via a command control circuit 311 and abuffer circuit 312. The command control circuit 311 outputs the commandsignal when either the chip select signal CS1B or CS2B is activated. Theaddress terminals 302 are supplied with an address signal via an addresscontrol circuit 321 and a buffer circuit 322. The address controlcircuit 321 outputs the address signal when either the chip selectsignal CS1B or CS2B is activated.

The chip select signals CS1B and CS2B are supplied from a chip selectcircuit 331. When the select circuit 331 activates the chip selectsignal CS1B, the chip select signal CS1B is supplied to thesemiconductor memory device 100 via the chip select terminal 303-1. Whenthe select circuit 331 activates the chip select signal CS2B, the chipselect signal CS2B is supplied to the semiconductor memory device 100via the chip select terminal 303-2.

When the select circuit 331 activates the chip select signal CS1B, adata input/output buffer 341 is activated. When the data input/outputbuffer 341 is activated, the data input/output terminals 304LDQ canreceive write data from the data input/output buffer 341 or receive readdata from the semiconductor memory device 100.

When the select circuit 331 activates the chip select signal CS2B, adata input/output buffer 342 is activated. When the data input/outputbuffer 342 is activated, the data input/output terminals 304UDQ canreceive write data from the data input/output buffer 341 or receive readdata from the semiconductor memory device 100.

Therefore, when the chip select terminal 303-1 receives an activatedchip select signal CS1B and the chip select terminal 303-2 receives aninactivated chip select signal CS2B, the data input/output terminals304LDQ receive read data or write and the input/output terminals 304UDQdo not receive the data. Similarly, when the chip select terminal 303-2receives an activated chip select signal CS2B and the chip selectterminal 303-1 receives an inactivated chip select signal CS1B, the datainput/output terminals 304UDQ receive read data or write and theinput/output terminals 304LDQ do not receive the data.

As described the above, the data input/output buffers 341 and 342 arecontrolled based on the chip select signals CS1B and CS2B. The commandterminals 301 and the address terminals 302 are provided in common tothe first group constituted of the chip select terminal 303-1 and thedata input/output terminals 304LDQ and the second group constituted ofthe chip select terminal 303-2 and the data input/output terminals304UDQ.

With this configuration, the controller 210 can obtain read data fromthe semiconductor memory device 100 or write data into the semiconductormemory device 100, by supplying the address signal ADD and the like viathe command/address bus 220. The controller 210 can individually accessthe plural memory circuit units 110A and 110B included in thesemiconductor memory device 100, by supplying the plural chip selectionsignals CS1B and CS2B to one semiconductor memory device 100.Consequently, the controller does not need to perform a process ofinvalidating unnecessary data.

FIG. 4 is a timing diagram for explaining a read operation of thesemiconductor memory device 100 according to the first embodiment, andshows a case of alternately accessing the memory circuit units 110A and110B.

In an example shown in FIG. 4, a mode-register set command MRS is issuedsynchronously with an active edge #0 of the external clock CK, therebysetting a burst length=4 (BL=4) and CAS latency=4 (CL=4) to the moderegister 125. Next, the active command ACT and the row address RA areinput synchronously with an active edge #2 of the external clock CK.During this period, both of the chip selection signals CS1B and CS2B areactivated at a low level. Therefore, the row address RA is latched byboth of the memory circuit units 110A and 110B. In FIG. 4, “no operation(NOP) command” is input during a period when the chip selection signalCS1B or CS2B is at a low level and also when a command is not written.The no operation (NOP) command is not shown in FIG. 4, and also notshown in other timing diagrams.

Next, a read command READ and a column address CA-A are inputsynchronously with an active edge #4 of the external clock CK in a statethat the chip selection signal CS2B is inactivated at a high level.Consequently, the read command READ and the column address CA-A arelatched by the memory circuit unit 110A, but are not latched by thememory circuit unit 110B. Therefore, only the memory circuit unit 110Aperforms a read operation, and starts burst output from an active edge#8 when the CAS latency (CL=4) passes. Because a DDR synchronous DRAM isassumed for the semiconductor memory device 100 according to the firstembodiment, one-bit read data is output at each half-clock cycle at aburst output time. Consequently, burst output of four bits started fromthe active edge #8 is completed at an active edge #10 (A0 to A3).

On the other hand, after the active edge #4 of the external clock CKpasses, the read command READ and a column address CA-B are inputsynchronously with an active edge #5 of the external clock CK in a statethat the chip selection signal CS1B is inactivated at a high level.Consequently, the read command READ and the column address CA-B arelatched by the memory circuit unit 110B, but are not latched by thememory circuit unit 110A. Therefore, only the memory circuit unit 110Bperforms a read operation, and starts burst output from an active edge#9 when the CAS latency (CL=4) passes. Burst output of four bits startedfrom the active edge #9 is completed at an active edge #11 (B0′ to B3′).

When the chip selection signals CS1B and CS2B are alternately activatedin this way, the memory circuit units 110A and 110B can be alternatelycontinuously accessed. That is, when the chip selection signals CS1B andCS2B are alternately activated, a shortest input cycle tCCD of a columncommand becomes BL/4 (=1), and the column command READ can be issued ateach one clock cycle. With this arrangement, as shown in FIG. 9, whenone of 64-bit data output from DQ0 to DQ15 and 64-bit data output fromDQ16 to DQ31 is necessary data and also when the other 64-bit data isunnecessary data, only the necessary data can be continuously taken out.Accordingly, utilization efficiency of a data bus can be improved.

FIG. 5 is a timing diagram for explaining a read operation of thesemiconductor memory device 100 according to the first embodiment, andshows a case of simultaneously accessing the memory circuit units 110Aand 110B.

In the example shown in FIG. 5, the mode-register set command MRS isalso issued synchronously with the active edge #0 of the external clockCK, thereby setting the burst length=4 (BL=4) and CAS latency=4 (CL=4)to the mode register 125. Next, the active command ACT and the rowaddress RA are input synchronously with the active edge #2 of theexternal clock CK. During this period, both of the chip selectionsignals CS1B and CS2B are activated at a low level. Therefore, the rowaddress RA is latched by both of the memory circuit units 110A and 110B.

Next, the read command READ and the column address CA-A are inputsynchronously with the active edge #4 of the external clock CK in astate that both of the chip selection signals CS1B and CS2B areactivated at a low level. Consequently, the read command READ and thecolumn address CA-A are latched by both of the memory circuit units 110Aand 110B, and the memory circuit units 110A and 110B simultaneouslyperform a read operation. Consequently, burst output is started from theactive edge #8 when the CAS latency (CL=4) passes. This burst output iscompleted at the active edge #10 (A0 to A3, A0′ to A3′).

Similarly, the read command READ and the column address CA-B are inputsynchronously with the active edge #6 of the external clock CK in astate that both of the chip selection signals CS1B and CS2B areactivated at a low level. Consequently, the memory circuit units 110Aand 110B simultaneously perform a read operation, and start burst outputfrom the active edge #10. This burst output is completed at an activeedge #12 (B0 to B3, B0′ to B3′).

When both of the chip selection signals CS1B and CS2B are activated inthis way, the shortest input cycle tCCD of a column command becomes BL/2(=2), and the semiconductor memory device can perform the same operationas that of a general semiconductor memory device. Therefore, thesemiconductor memory device can maintain compatibility with existingsemiconductor memory devices.

While a read operation in the first embodiment has been explained above,the above explanations are also applied to a write operation. That is,the memory circuit unit 110A can perform a write operation via the datainput/output terminal group LDQ regardless of an operation of the memorycircuit unit 110B, and the memory circuit unit 110B can perform a writeoperation via the data input/output terminal group UDQ regardless of anoperation of the memory circuit unit 110A.

FIG. 6 is a timing diagram for explaining a refresh operation of thesemiconductor memory device 100 according to the first embodiment.

In an example shown in FIG. 6, a total-bank precharge command PALL isissued synchronously with the active edge #0 of the external clock CK,and further a refresh command REF is issued synchronously with theactive edges #2, #5, and #8. During this period, both of the chipselection signals CS1B and CS2B are activated at a low level. Therefore,the refresh command REF is valid in both of the memory circuit units110A and 110B, and a refresh operation is performed simultaneously inthe memory circuit units 110A and 110B. In this way, the semiconductormemory device 100 according to the first embodiment can perform arefresh operation similar to that of a general DRAM.

A second embodiment of the present invention is explained next.

FIG. 7 is a block diagram showing a configuration of a semiconductormemory device 300 according to the second embodiment.

The semiconductor memory device 300 according to the second embodimentis different from the semiconductor memory device 100 in that two moderegisters 125 are provided. Other features of the semiconductor memorydevice 300 are identical to those of the semiconductor memory device 100described above, therefore like reference characters are denoted to likeelements and redundant explanations thereof will be omitted.

Mode registers 125A and 125B included in the semiconductor memory device300 according to the second embodiment are circuits to set operationmodes of the memory circuit units 110A and 110B, respectively. That is,in the second embodiment, an operation mode of the memory circuit unit110A and an operation mode of the memory circuit unit 110B can be setseparately.

FIG. 8 is a timing diagram for explaining a read operation of thesemiconductor memory device 300 according to the second embodiment, andshows a case of alternately accessing the memory circuit units 110A and110B.

In an example shown in FIG. 8, the mode-register set command MRS isissued synchronously with an active edge #-1 of the external clock CK ina state that the chip selection signal CS1B is activated, therebysetting the burst length=4 (BL=4) and CAS latency=5 (CL=5) to the moderegister 125A. Further, the mode-register set command MRS is issuedsynchronously with the active edge #0 of the external clock CK in astate that the chip selection signal CS2B is activated, thereby settingthe burst length=4 (BL=4) and CAS latency=4 (CL=4) to the mode register125B. In this way, mutually different CAS latencies are set to the moderegisters 125A and 125B.

Thereafter, a read operation is similar to the read operation shown inFIG. 4. The active command ACT and the row address RA are inputsynchronously with the active edge #2 of the external clock CK, andthereafter, the read command READ is issued at each one clock cycle(=BL/4) while alternately activating the chip selection signals CS1B andCS2B.

As a result, the read command READ issued at the active edge #4 becomesvalid in the memory circuit unit 110A, and starts burst output from theactive edge #9 when the CAS latency (CL=5) passes. On the other hand,the read command READ issued at the active edge #5 becomes valid in thememory circuit unit 110B, and starts burst output from the active edge#9 when the CAS latency (CL=4) passes. That is, burst output can besimultaneously performed from the data input/output terminal groups LDQand UDQ.

As described above, in the semiconductor memory device 300 according tothe second embodiment, because the operation mode of the memory circuitunit 110A and the operation mode of the memory circuit unit 110B can beset separately, read data can be output simultaneously while alternatelyissuing mutually different read commands to the memory circuit units110A and 110B. Accordingly, the controller can easily handle the readdata.

It is apparent that the present invention is not limited to the aboveembodiments, but may be modified and changed without departing from thescope and spirit of the invention.

For example, in the above embodiments, although inside of each of thesemiconductor memory devices 100 and 300 is divided into the two memorycircuit units 110A and 110B, the number of division is not limited totwo, and the inside can be divided into three or more memory circuitunits.

In the above embodiments, although the command decoder 124 is providedin the common circuit 120, either a part or the whole of the commanddecoder 124 can be also provided in the memory circuit units 110A and110B. Therefore, a command latched by the command latch circuits 112CMAand 112CMB can be a decoded command or an undecoded command.

In the above embodiments, although the chip selection signals CS1B andCS2B are external signals, it is not essential that the chip selectionsignal itself is an external signal. For example, an internal signalobtained by decoding a binary signal constituted by plural bits can beused as a chip selection signal.

Furthermore, the present invention can be also applied to asemiconductor memory device having a memory cell array divided intoplural banks. FIG. 10 is a block diagram of a semiconductor memorydevice having plural banks. The semiconductor memory device shown inFIG. 10 includes four banks BANK0 to BANK3, and each of the banksincludes the memory circuit units 110A and 110B. In this manner, asemiconductor memory device having plural banks can have plural memorycircuit units in each bank.

In addition, while not specifically claimed in the claim section, theapplicant reserves the right to include in the claim section of theapplication at any appropriate time the following devices:

A1. A semiconductor device comprising:

a memory cell array including a plurality of memory cells;

a plurality of data input/output terminals; and

a plurality of address terminals, wherein

during a period when a group of data corresponding to first addressinformation supplied from the address terminals is transmitted orreceived by using data input/output terminals of which number is smallerthan number of the data input/output terminals, a group of datacorresponding to second address information supplied from the addressterminals is transmitted or received by using rest of the datainput/output terminals.

A2. A semiconductor device formed on a single semiconductor substrate,the semiconductor memory device comprising:

an address/command terminal group that receives address information andcommand information;

first and second memory circuit units;

first and second data input/output terminal groups providedcorresponding to the first and second memory circuit units,respectively;

a selected-information input terminal group; and

a control circuit connected to the address/command terminal group andthe selected-information input terminal group, wherein

the control circuit performs a first data transfer operation between thefirst memory circuit unit and the first data input/output terminal groupbased on the address information and the command information wheninformation from the selected-information input terminal group selectsthe first memory circuit unit,

the control circuit performs a second data transfer operation betweenthe second memory circuit unit and the second data input/output terminalgroup based on the address information and the command information whenthe information from the selected-information input terminal groupselects the second memory circuit unit, and

the control circuit performs the first and second data transferoperations when the information from the selected-information inputterminal group selects both of the first and second memory circuitunits.

1. A semiconductor device comprising: a semiconductor substrate, firstand second data input/output terminals provided on the semiconductorsubstrate, first and second memory circuit units provided on thesemiconductor substrate, the first memory circuit unit being activatedin response to a first selection signal so that the first memory circuitunit inputs/outputs data from/to the first data input/output terminaland does not input/output data from/to the second data input/outputterminal, the second memory circuit unit being activated in response toa second selection signal so that the second memory circuit unitinputs/outputs data from/to the second data input/output terminal anddoes not input/output data from/to the first data input/output terminal,a command terminal provided in common to the first and second memorycircuit units, wherein the first and second memory circuit unitsrespectively comprise first and second command latch circuits that latcha command signal supplied via the command terminal or latch a signalobtained by decoding the command signal, a latch operation performed bythe first command latch circuit is permitted when the first selectionsignal is activated, and a latch operation performed by the secondcommand latch circuit is permitted when the second selection signal isactivated.
 2. The semiconductor device as claimed in claim 1, furthercomprising first and second selection-signal input terminals to whichthe first and second selection signals are supplied, respectively. 3.The semiconductor device as claimed in claim 1, further comprising amode register that sets operation modes of the first and second memorycircuit units common to each other.
 4. The semiconductor device asclaimed in claim 1, further comprising a first mode register that setsan operation mode of the first memory circuit unit, and a second moderegister that sets an operation mode of the second memory circuit unit.5. The semiconductor device as claimed in claim 1, further comprising anaddress terminal provided in common to the first and second memorycircuit units, wherein when the first selection signal is activated, thefirst memory circuit unit performs a read operation or a write operationvia the first data input/output terminal based on an address signalsupplied via the address terminal, regardless of an operation of thesecond memory circuit unit, and when the second selection signal isactivated, the second memory circuit unit performs a read operation or awrite operation via the second data input/output terminal based on theaddress signal, regardless of an operation of the first memory circuitunit.
 6. The semiconductor device as claimed in claim 5, wherein thefirst and second memory circuit units respectively comprise first andsecond address latch circuits that latch the address signal or a signalobtained by decoding the address signal, a latch operation performed bythe first address latch circuit is permitted when the first selectionsignal is activated, and a latch operation performed by the secondaddress latch circuit is permitted when the second selection signal isactivated.
 7. The semiconductor device as claimed in claim 1, whereinwhen a refresh command is supplied via the command terminal in a statethat the first selection signal is activated, the first memory circuitunit performs a refresh operation regardless of an operation of thesecond memory circuit unit, and when the refresh command is supplied ina state that the second selection signal is activated, the second memorycircuit unit performs a refresh operation regardless of an operation ofthe first memory circuit unit.
 8. The semiconductor device as claimed inclaim 1, further comprising a DLL circuit provided in common to thefirst and second memory circuit units, wherein the DLL circuit controlsan output timing of read data via the first data input/output terminalby the first memory circuit unit, and an output timing of read data viathe second data input/output terminal by the second memory circuit unit.9. A semiconductor device comprising: a semiconductor substrate, firstand second data input/output terminals provided on the semiconductorsubstrate, first and second memory circuit units provided on thesemiconductor substrate, the first memory circuit unit being activatedin response to a first selection signal so that the first memory circuitunit inputs/outputs data from/to the first data input/output terminaland does not input/output data from/to the second data input/outputterminal, the second memory circuit unit being activated in response toa second selection signal so that the second memory circuit unitinputs/outputs data from/to the second data input/output terminal anddoes not input/output data from/to the first data input/output terminal,an address terminal provided in common to the first and second memorycircuit units, wherein when the first selection signal is activated, thefirst memory circuit unit performs a read operation or a write operationvia the first data input/output terminal based on an address signalsupplied via the address terminal, regardless of an operation of thesecond memory circuit unit, and when the second selection si nal isactivated the second memor circuit unit performs a read operation or awrite operation via the second data input/output terminal based on theaddress signal, regardless of an operation of the first memory circuitunit, wherein in response to an input of a second address signal in astate that the second selection signal is activated, after a firstperiod subsequent to a timing when a first address signal is supplied ina state that the first selection signal is activated, the first memorycircuit unit starts a read operation or a write operation correspondingto the first address signal via the first data input/output terminal,and the second memory circuit unit starts a read operation or a writeoperation corresponding to the second address signal via the second datainput/output terminal, at substantially a same timing as a start of theread operation or the write operation by the first memory circuit unit.10. A semiconductor device comprising: a semiconductor substrate; firstand second selection-signal input terminals each provided on thesemiconductor substrate, the first and second selection-signal inputterminals being supplied with first and second selection signals,respectively; first and second command circuits each provided on thesemiconductor substrate, the first command circuit latching a firstwrite/read command signal when the first selection signal is activatedand the second command circuit latching a second write/read commandsignal when the second selection signal is activated; and first andsecond memory circuit units each provided on the semiconductorsubstrate, the first memory circuit unit performning a write/readoperation in response to the first write/read command signal latched inthe first command circuit and the second memory circuit unit performinga write/read operation in response to the second write/read commandsignal latched in the second command circuit.
 11. The semiconductordevice as claimed in claim 10, further comprising first and second datainput/output terminals each provided on the semiconductor substrate,the' first memory circuit unit inputting/outputting data from/to thefirst data input/output terminal and not inputting/outputting datafrom/to the second data input/output terminal, the second memory circuitunit inputting/outputting data from/to the second data input/outputterminal and not inputting/outputting data from/to the first datainput/output terminal.
 12. The semiconductor device as claimed in claim10, further comprising a first mode register that sets an operation modeof the first memory circuit unit and a second mode register that sets anoperation mode of the second memory circuit unit.
 13. The semiconductordevice as claimed in claim 12, wherein the first mode register sets theoperation mode of the first memory circuit unit when the first selectionsignal is activated and the second mode register sets the operation modeof the second memory circuit unit when the second selection signal isactivated.
 14. The semiconductor device as claimed in claim 10, whereinthe first command circuit latches a first refresh command signal whenthe first selection signal is activated and the second command circuitlatches a second refresh command signal when the second selection signalis activated, the first memory circuit unit performing a refreshoperation in response to the latched first refresh command signal andthe second memory circuit unit performing a refresh operation inresponse to the latched second refresh command signal.